The invention relates generally to a combined cycle operation and more particularly, to a method and system for controlling a drum water level during operation of a heat recovery steam generation (HRSG) system for a combined cycle power plant.
Drum type boilers are the most commonly used boilers in the combined cycle power plants. In power generation industry, drum level trips due to ineffective drum level control response during transient operations are a primary cause of disruption in power generation and lead to a huge loss of plant availability and revenue.
Control of drum water level is a challenging problem due to complicated dynamics of two-phase flows, presence of waves, unknown heat and pressure disturbances, and load demands. Maintaining water level within limits is critical, as exceeding the limits will lead to trips or damage the equipment. A drop in water level will cause thermal fatigue in the drum. An increase in water level significantly increases the possibility of water droplets entering a superheater/steam turbine and thus damaging the superheater/steam turbine.
Conventional approaches to control of water level typically include actuation of a feed water control valve that supplies water to the drum in reaction to observed changes in level and steam flowrate. However, such approaches to control water level are challenging for drum type boilers, especially during transient operating conditions due to inverse response exhibited by such systems. The disturbance rejection responses of level controllers of existing systems are inadequate to handle such pressure disturbances arising from bypass operations.
There is therefore a need for a more effective technique for controlling a liquid level in a vessel, such as for example, control of liquid level in a drum type boiler, especially during transient operating conditions.